Description of proposed changes

To date, we have been able to plot (draw or fill) DCW countries or dump their coordinates to stdout. This PR adds clipping to the mix. With the new modifiers +c and +C you can use coast to set up clip paths that will affect the following plotting commands until you terminate clipping with clip -C. There are two flavors to the clipping option:

1. +c will set up clip paths so the things plotted inside the paths will be visible.
2. +C will set up clip paths so the things plotted outside the paths (i.e., between the map boundary and the paths) will be visible.

I have added a test script for this (coastclip.sh) which sets up these paths and then tries to paint the entire region using basemap -B+g: